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Pre-Processing
Simcenter STAR-CCM+ solves physics problems on a computational mesh. To provide this mesh, you first define geometry using internal or external tools, and then create a mesh pipeline whose operations generate the mesh in repeatable steps.
Meshing
A mesh is a discretized representation of a geometric domain. This domain can include real-world geometry, its content, and its surrounding environment.
Checking the Volume Mesh
This section describes the tools available for checking the validity and quality of the volume mesh, and for diagnosing problem areas. Such problems can then be fixed, either inside Simcenter STAR-CCM+ or in the originating mesh generator.
What Is a Valid Mesh?
Fundamentally, a valid mesh is one that allows the simulation to be initialized and then run so that a solution is achieved. An invalid mesh is typically one for which no solution can be obtained or cannot even be initialized. As an example of the latter situation, a mesh that contains a negative volume cell would be considered invalid.

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Pre-Processing
Simcenter STAR-CCM+ solves physics problems on a computational mesh. To provide this mesh, you first define geometry using internal or external tools, and then create a mesh pipeline whose operations generate the mesh in repeatable steps.
- 3D-CAD
  3D-CAD is a feature-based parametric modeler within Simcenter STAR-CCM+ that allows you to build geometries from scratch. The geometries that you create with 3D-CAD are stored as 3D-CAD models, which you can then convert to geometry parts for integration with the meshing and simulation process.
- Modeling Geometry
  This part of the User Guide describes how to use 3D-CAD, the parametric solid modeler available within Simcenter STAR-CCM+. It also describes the features and tools available for further preparation of the geometry model and its surfaces.
- The Surface Repair Tool
  The Surface Repair tool diagnoses errors in, and provides repair tools for, tessellated surfaces and part curves. The same tool allows you to reorganize discretized geometry by searching for face-based features and attributes then assigning the results to new or existing part surfaces or curves.
- Defining the Regions Layout
  An essential part of the process in setting up your simulation is to define the relationship between geometry parts and regions, boundaries, and interfaces.
- Meshing
  A mesh is a discretized representation of a geometric domain. This domain can include real-world geometry, its content, and its surrounding environment.
  - Mesh Construction
    Within a bounding volume defined by geometrical surfaces, Simcenter STAR-CCM+ can construct meshes for finite volume and finite element solvers.
  - Meshers in Simcenter STAR-CCM+
    The meshers in Simcenter STAR-CCM+ generate surface and volume meshes for geometry parts that are assigned to regions and make them ready for analysis.
  - Parts-Based Meshing
    Parts-based meshing (PBM) is a meshing strategy in which you create one or more mesh operations that define workflows for preparing, fixing, and processing geometry. A major benefit of PBM is that every mesh operation you create forms part of a linear and repeatable process.
  - The Meshing Workflow
    This section contains the recommended workflow for setting up and generating the volume mesh at the parts level.
  - Mesh Operations
    Mesh operations perform actions on geometry parts that you can easily repeat whenever their input parts or properties are changed.
  - Mesh Descriptions
    A mesh description is a version of a surface or volume mesh belonging to a geometry part. Geometry parts can have more than one surface mesh description associated with them. You create and change descriptions by importing surfaces or executing operations.
  - Custom Mesh Controls
    Custom controls override any default controls for the surface and volume meshers. This facility allows you to refine or coarsen the mesh for part curves, part surfaces, geometry parts, and specified volumes.
  - Surface Meshers
    Simcenter STAR-CCM+ contains meshing tools which can be used to help prepare a high-quality surface mesh before a volume mesh can be generated.
  - Volume Meshers
    Simcenter STAR-CCM+ contains different types of meshers that can be used to generate a volume mesh starting from a suitably prepared surface mesh.
  - Mesh Refinement
    Simcenter STAR-CCM+ can refine the mesh by approximating the wake region or using solution-based approach. These features allow you to achieve efficient refinement by reducing the cell size in areas of interest.
  - Directed Meshing
    Directed meshing is a method for generating high quality swept meshes on geometries in Simcenter STAR-CCM+. The method operates by sweeping a mesh from the surfaces of a geometry through its volume onto a facing target surface.
  - Two Dimensional and Axisymmetric Meshes
    The use of a one-cell-thick three-dimensional mesh is much less efficient than using a real two-dimensional mesh for two-dimensional and axisymmetric simulations. When circumstances allow, use real two-dimensional meshes.
  - Turbomachinery Mesh
    Turbomachinery applications with axial blades typically require a flow-aligned, structured mesh. This type of mesh has implicit connectivity and a fixed number of vertex or face neighbors, leading to a more accurate solution.
  - Mesh Visualization
    You can visualize any mesh representation in Simcenter STAR-CCM+. After you have imported or generated a mesh, you visualize it to check the cell quality and mesh validity.
  - Checking the Surface Mesh
    The volume mesh quality is directly dependent on the surface mesh quality. Therefore, it is crucial to make sure that all surface meshes are of good quality.
  - Checking the Volume Mesh
    This section describes the tools available for checking the validity and quality of the volume mesh, and for diagnosing problem areas. Such problems can then be fixed, either inside Simcenter STAR-CCM+ or in the originating mesh generator.
    - What Is a Valid Mesh?
      Fundamentally, a valid mesh is one that allows the simulation to be initialized and then run so that a solution is achieved. An invalid mesh is typically one for which no solution can be obtained or cannot even be initialized. As an example of the latter situation, a mesh that contains a negative volume cell would be considered invalid.
    - Mesh Quality
      Simcenter STAR-CCM+ provides a number of cell metrics that you can use to determine the overall quality of the volume mesh. In general, a poor quality volume mesh does not necessarily result in solver run time problems, but it does reduce the accuracy and efficiency of the solution obtained.
    - Running Mesh Diagnostics
      This section describes how to run a mesh diagnostic report and how to interpret the output.
    - Checking The Mesh Validity
      The mesh validity is automatically checked whenever a mesh diagnostics report is produced or when the simulation is initialized for the first time.
    - Comparing a Volume Mesh with the Geometry
      By displaying two different mesh representations, you can compare the volume mesh with the geometry surface to determine the accuracy of the discretization.
    - Using The Connected Cells Field Function
      When the volume mesh generation is complete, Simcenter STAR-CCM+ exposes a field function, Connected Cells, that has the same value for any pair of cells that share a face (interior or interface).
    - Checking The Skewness Angle
      The Cell skewness angle and boundary skewness angle are important measures of cell quality. This section describes the meaning of the terms and how to find the worst (highest) values in the mesh diagnostic report.
    - Identifying And Visualizing Bad Cells
      To identify and visualize cells in Simcenter STAR-CCM+ according to a given criteria, such as one of the quality metrics, use threshold values.
    - Removing Invalid Cells
      Poor quality cells can cause the solution to diverge or compromise the accuracy of the results. Simcenter STAR-CCM+ provides a tool for removing poor quality mesh cells based on specified mesh criteria.
  - Meshing Field Functions Reference
    This section contains a list of field functions that are related to meshing.
  - Working With Representations
    This section provides information on representations as a useful part of the meshing process.
  - Scaling the Entire Mesh
    Scaling the mesh is important as the units of the model must be in meters before running an analysis.
  - Working with Baffles
  - Exporting Mesh and Solution Data
    This section outlines the steps for exporting Simcenter STAR-CCM+ mesh and solution data to formats that can be used in other CAE packages.
  - Multi-Region Meshing
    This section outlines the surface requirements for performing multi-region meshing, as well as step-by-step instructions when starting from surface imports that result in single region and multiple regions definitions. Additionally, guidelines are provided on what the best approaches are for exporting surface data from CAD packages when multiple regions are required.

What Is a Valid Mesh?

Fundamentally, a valid mesh is one that allows the simulation to be initialized and then run so that a solution is achieved. An invalid mesh is typically one for which no solution can be obtained or cannot even be initialized. As an example of the latter situation, a mesh that contains a negative volume cell would be considered invalid.

Based on that definition, the determination of whether a mesh is valid is not only dependent on the underlying mesh quality, but also the physics models in use with the mesh. For example, meshes with cells with high concave angles would not normally cause a problem, but in cases with high gradients or complex physics, a solution may not be obtainable. In this case, the mesh is considered invalid.

Other factors that can affect whether a mesh is valid include:

choice of solver (coupled or segregated)
under-relaxation or Courant number specification; and
discretization scheme that is used

The approach that is used to define a CFD analysis is also important. For example, adding the physical models in one by one, iterating and restarting for each might lead to a valid solution. However, adding in all the physical models at once and attempting to run the simulation might cause the solution to diverge for a given mesh.

The mesh validity checks that are performed in Simcenter STAR-CCM+ are: